1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic system and a developing device used for the image forming apparatus.
2. Description of the Related Art
As a dry-type developing system applied to an electrophotographic system, a one-component developing system using only a toner and a two-component developing system using a developer containing a toner and a magnetic carrier are known.
In the one-component developing system, since there is no magnetic carrier, an electrostatic image of an image bearing member is not disturbed by a magnetic brush formed from a magnetic carrier, and the system is suitable for obtaining a high-quality image. However, in the one-component developing system, it is difficult to stably provide charges to the toner and there is a problem in the stability of the image quality. In addition, since there is no medium for transporting the toner, such as a magnetic carrier, it is difficult to exert a uniform transport force to the toner, and a mechanical load to the toner is easily increased during transporting or the like. Therefore, degradation in the stability of the image quality easily occurs due to the deterioration of the toner.
On the other hand, in the two-component developing system, although there is a problem in the image quality, since the toner is easily provided with charges and the load to the toner is small, the two-component developing system has a feature in that the stability of the image quality is high.
As a system of coping with the problems of the above-described two developing systems, there is known a hybrid developing system disclosed in, for example, Japanese Patent Laid-Open No. 9-211970. In this system, an image is formed by applying a transporting bias between a transporting roll (developer carrying member) which carries two-component developer and a developing roll (toner carrying member), coating the developing roll with a toner layer, and developing an electrostatic image of a photosensitive member (image bearing member) by using the toner.
However, it is known that, in the hybrid developing system, it is difficult to coat the developing roll with a stable toner layer for a long term. In the hybrid developing system, the developing roll is coated with the toner having a predetermined charge amount (Q/S) so that a potential difference ΔV generated by the above-described transporting bias is filled between the transporting roll and the developing roll. In this case, there is a relationship where ΔV and the charge amount Q/S of the toner per unit area to be coated are proportional to each other. In addition, Q/S is a product of the mass (M/S) of the toner involved with the coat per unit area and the charge amount (Q/M) per unit mass of the toner.
Therefore, the following equation is obtained.ΔV∝Q/S=(M/S)×(Q/M)  Equation (1)
In other words, in the hybrid developing system, the mass (M/S) of the toner involved with the coat per unit area is determined based on the potential difference (ΔV) and the charge amount (Q/M) per unit mass of the toner. Therefore, the hybrid developing system has a problem in that, if the charging amount of the toner is changed, the toner amount involved with the coat is varied according to the change.
With respect to this problem, for example, Japanese Patent Laid-Open No. 2009-8834 discloses a configuration of measuring a thickness of a toner layer on a developing roll by using a toner layer thickness sensing member when coating the developing roll with the toner layer. In addition, the patent document also discloses a configuration of controlling the thickness of the toner layer on the developing roll to be a predetermined layer thickness by changing a transport bias between the developing roll and a magnetic roll (developer carrying member) or the number of rotations of the developing roll and the magnetic roll based on the thickness of the toner layer.
However, in the configuration, since a toner density sensor or a surface potential sensor is used as the toner layer thickness sensing member, the size of the device becomes large, or the cost is increased. In addition, even in the case of performing control by using a sensing member, if the transporting bias or the number of rotations of the developing roll is changed, since the developing conditions between the developing roll and the photosensitive member in the downstream also needs to be controlled simultaneously, the control becomes complicated. As a result, there is a problem in that it is difficult to achieve the original purpose of stabilizing the toner amount on the photosensitive member.
Therefore, as a developing system of coating a stable toner layer, for example, Japanese Patent Laid-Open No. 10-198161 discloses a configuration of using a rotatable regulating sleeve (developer regulating member) which is arranged to be separated by a certain interval from a developing roll. As a result, the toner is stably provided with charges by a carrier, so that the developing roll can be coated with a toner layer without a decrease in output image density or scattering of the toner. The developing device 120 is configured to include a developer container 121 which contains a developer 110 including a toner and a magnetic carrier.
Hereinafter, the developing device 120 will be described with reference to FIG. 22.
A developing roll 122, which is rotatable in the arrow direction of FIG. 22, and a carrier recovering member 123, which is separated by a certain distance above the developing roll 122, are arranged in an aperture of the developer container 121 which is formed at a position where the developer container faces the photosensitive member 101. The carrier recovering member 123 is configured to include a regulating sleeve 231 which is a non-magnetic member and a permanent magnet 232 which is arranged to be fixed inside thereof, and the regulating sleeve 231 is rotatably carried in the same direction as the rotational direction (arrow direction of FIG. 22) of the developing roll 122. In addition, a transporting member 124 which stirs the developer in the developer container 121 and supplies the developer to the developing roll 122 through the rotation (arrow direction of FIG. 22) is installed in the developer container 121.
Next, the coating of the toner layer on the developing roll 122 in the developing device 120 will be described.
The developer 110 in the developer container 121 is stirred and supplied to the developing roll 122 simultaneously by the transporting member 124. The to-be-supplied developer 110 is carried by the developing roll 122 which is magnetized by exertion of a magnetic force of a permanent magnet 232 in the regulating sleeve 231 to be transported and is regulated in the developer regulation region G.
FIG. 23 is an enlarged diagram of the developer regulation region G.
The magnetic carrier in the developer, restrained by the magnetic field in the developer regulation region G, is restrained by the magnetic force of the permanent magnet 232. Since regulating sleeve 231 is rotated in the arrow direction of the FIG. 23, the magnetic carrier has a transporting force exerted on it in the direction (direction A of FIG. 23) where the magnetic carrier is to be returned to the developer container 121 according to the rotation. Therefore, since the magnetic carrier is restrained in the developer regulation region G, the magnetic carrier is sequentially returned to the developer container 121 by the transporting force from the regulating sleeve 231 and the magnetic carrier does not leak out to the developing portion facing the photosensitive member 101.
On the other hand, the non-magnetic toner 111 in the developer in the developer regulation region G is not restrained by the magnetic field in the developer regulation region G. In addition, the non-magnetic toner 111 is adhered to the developing roll 122 by a reflection force generated by the charges provided by frictional charging between the magnetic carrier and the surface of the developing roll 122. Therefore, the non-magnetic toner 111 has a transport force exerted on it in the rotational direction (direction B of FIG. 23) of the developing roll 122 and according to the rotation of the developing roll 122 is caused to pass through the developer particles in the developer regulation region G to coat the developing roll 122.
As described above, the developing roll 122 may be coated with only the non-magnetic toner provided with sufficient charges without leakage of the magnetic carrier in the developing portion. According to the configuration disclosed in Japanese Patent Laid-Open No. 10-198161, since a force exerted on the toner which can be physically in contact with the developing roll is used, the phenomenon observed in the hybrid developing system that the toner amount involved with the coating is rapidly changed due to the change in charge amount (Q/M) of the toner does not occur.
In this manner, in the case where the charge amount of the toner is decreased, in the device of the hybrid developing system, the toner amount involved with the coat is increased. However, in the device disclosed in Japanese Patent Laid-Open No. 10-198161, since the increase in toner amount involved with the coat is suppressed, the change of the image density caused by the increase in toner amount can be suppressed.
However, it is found out from a result of detailed examination by the inventors of the present invention that, even in the developing device disclosed in Japanese Patent Laid-Open No. 10-198161, it is necessary to further suppress the change of the image density and to further improve image uniformity.
FIG. 24 is a conceptual diagram illustrating a toner layer obtained by the developing device 120 where the developing roll is coated with the toner layer. Black portions represent portions of the coated toner layer, and white portions represent areas which are not coated with the toner. As illustrated in FIG. 24, the areas which are not coated with the toner irregularly exist substantially in parallel to the rotational direction of the developing roll, and the toner density on the developing roll is non-uniform. In this manner, if the coating layer by the toner is formed non-uniformly on the developing roll, the image density is easily decreased. This is because the area of the white sheet portion where the sheet cannot be coated with the toner is increased during fixing, so that the image density is rapidly decreased.
On the other hand, more toner can be supplied to the photosensitive member by adjusting circumferential velocities of the developing roll and the photosensitive member, so that the image density can be increased. More specifically, in the case where the developing roll and the photosensitive member are rotated in the same direction in the facing portion, the increase of the image density can be achieved by allowing the circumferential velocity of the developing roll to be higher than that of the photosensitive member or by allowing the rotational directions of the developing roll and the photosensitive member to be reverse to each other in the facing portion. However, although a desired image density is obtained in this manner, as illustrated in FIG. 25B, only an image where the in-plane density irregularity is conspicuous and the image uniformity is low is obtained. In addition, in terms of reduction of energy consumption, it is required to output a desired image with a smaller toner amount. However, it denotes that more toner than required is consumed.
FIG. 25A is a schematic diagram illustrating the case where the electrostatic image on the photosensitive member is ideally developed with the toner. In addition, FIG. 25B is a schematic diagram illustrating the case where the image density is obtained by the above-described method.
In FIG. 25A, a toner image having a high uniformity can be obtained with a small toner amount. However, in FIG. 25B, the toner amount is large, and a toner image has a low uniformity.
It has been found out from a result of detailed examination by the inventors of the present invention that the reasons for this phenomenon can be described by models described below. This will be described with reference to FIG. 26.
FIG. 26 illustrates a state where the developer 110, which is transported in the rotational direction h of the developing roll 122 constitutes a magnetic brush by a magnetic force in the developer regulation region G to be restrained in the carrier recovering member 123 and is transported in the rotational direction j of the carrier recovering member 123. In the actual case, a larger number of the developer particles than shown in FIG. 26 exist as the magnetic brush.
In the process where the developer 110 is transported on the developing roll 122, the toner 111 of the developer 110 is charged by being in contact with the developing roll 122. At this time, the toner 111 is separated from the magnetic carrier 112 and is adhered to the developing roll 122.
On the other hand, as described above, the developer 110 which is restrained by the carrier recovering member 123 is transported in the rotational direction j (from the downstream side in the rotational direction h). Since the toner 111 has already been consumed (removed) from the developer 110 at the upstream side in the rotational direction j, the magnetic carrier 112 in the developer 110 has a capability of recovering the toner. Therefore, if the developer 110 transported in the rotational direction j of the carrier recovering member 123 is in contact with the toner 111 adhered to the developing roll 122, the toner 111 is recovered by the magnetic carrier 112 to be returned to the developer container 121.
FIGS. 27A and 27B are schematic diagrams illustrating a state where the toner 111 attached on the developing roll 122 is recovered by the magnetic carrier 112 of the developer 110.
If the developer 110 collides with the toner 111 on the developing roll 122 (FIG. 27A), a couple of forces are exerted on the toner 111, so that the toner is rotated on the developing roll 122 (FIG. 27B). Therefore, the adhesion force between the toner and the developing roll is decreased. At this time, since the magnetic carrier 112 is charged corresponding to the charges of the consumed toner with the opposite-polarity, the toner which coats the developing roll is scraped off by the magnetic carrier 112 while passing through the developer regulation region G. It is found out that, in this manner, since a trace of scraping by the magnetic carrier easily occurs in the transporting direction of the developer 110, that is, in the direction substantially parallel to the rotational direction of, mainly, the developing roll or the carrier recovering member, it is not possible to form a uniform coating of a toner layer on the developing roll.